251.4 million years ago, Earth experienced the Permian Triassic Extinction Event, also called the Great Dying. No one really knows what caused it, but 70% of all terrestrial life and 96% of marine life was destroyed. Some scholars suspect it may have been a series of meteor impacts, or radiation when a nearby star became a quasar.
In the Mesozoic Era (251.4 - 65 million years ago) animal life was recovering from the worst mass extinction in the world's history. It is against this backdrop that dinosaurs developed and quickly spread across the planet.
During the first third of the Mesozoic Era, the Triassic Period (251.4 - 213 million years ago, see chart), true dinosaurs had not yet appeared. The Triassic hosted the appearance of archosaurs, which were a kind of muscular lizard.
Two important evolutionary changes took place among the archosaurs. They changed from sprawling, lizardlike animals to animals that walked with their legs held directly under their bodies. The ancestors of the dinosaurs developed a stronger ankle, which allowed then to walk upright and sometimes become bipedal (walking on two legs). The other change was from cold-blooded, lizardlike metabolism to a warm-blooded, birdlike metabolism.
True dinosaurs at last! During the Jurassic Period dinosaurs grew very large. The massive Sauropods, as large as 60 bull elephants, developed early in this period. Alongside them, in the middle and late Jurassic, evolved the carnivore giants.
Late Jurassic dinosaurs were spectacular. They exceeded the imagination in size and form. Finally, by the Cretaceous Period, dinosaurs ruled the Earth, and had spread all across the land. This continued until their mysterious mass extinction, 65 million years ago.
The earth was still changing as the early Cretaceous began. The land masses were drifting and the climate was changing. These changes affected the plant and animal world. Dinosaurs were becoming more plentiful and many more types were evolving. The Early Cretaceous world was very warm. There were wet and dry seasons rather than summer and winter. Most areas of the world were covered by tropical jungles. The low-growing plants were ferns and fernlike vegetation. A perfect breeding ground for dinosaurs, large and small.
If you're interested in this sort of thing, there are a lot of good books on the subject: some very scholarly and others more basic. One of my favorites is National Geographic Dinosaurs, by Paul Barrett.
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